1. Field of the Invention
The present invention is related to a design of a girder for a bridge or for use in construction, and more particularly, to a method for designing and fabricating a multi-step tension prestressed girder to increase a load bearing force of a bridge, when necessary, by adjusting tension step by step during construction.
2. Description of the Related Art
FIG. 1 shows the arrangement of steel wires between girders according to a conventional design. Referring to the drawing, a prestressed concrete girder 11 includes an upper flange 13, a lower flange 14, and a body 15. A steel wire 12 is installed lengthwise in the girder 11 from end to end through the body 15, near the lower flange 14. In the conventional girder design, girders having various profiles is have been developed to be sturdier and longer in an effort to improve the efficiency of a member. However, there have been no remarkable developments in basic methods of introducing prestress to the member and the conventional design methods have still been used. In the conventional design which is based on an allowed stress design concept, a girder is fabricated as a precast member in a factory or is directly fabricated at a construction site, and then a required tension in view of a designed load is initially applied once to the girder. The tension should be applied such that the prestress occurring at this time can be greater than a total bending stress generated in the girder due to a dead load and a live load added thereto. Also, since a tension process is performed only one time, great prestress, considering an overall loss of tension, needs to be introduced initially. Accordingly, the area and height of the profile of the girder should be initially sufficient to bear the prestress.
FIG. 2 shows a relationship between load and stress according to the conventional design method. Prestress generated by tension P1 introduced in a pretension or post-tension method after a girder has been fabricated is distributed as indicated by a line 1. However, such a state is theoretical. Actually, since a bending moment Md1 due to the weight of the girder itself exists prior to the introduction of tension, the distribution of stress in which the bending stress due to the self-weight and the prestress have been synthesized is shown by a line 2. Here, the tensile stress in an upper margin of the girder should not exceed σti and the compression stress in a lower margin of the girder should not exceed σci.
When loss of tension occurs by lapse of time in the stress distribution state of line 2, prestress is reduced so that the distribution of stress moves to a line 3. That is, tensile stress decreases by Δσ1 in the upper margin and compresion stress decreases by Δσ2 in the lower margin.
Here, when the additional dead load moment Md2 and the live load M1 are introduced, the distribution of stress becomes as shown by a line 4 of FIG. 2. The stress in the lower margin should not exceed σts and the stress in the upper margin should not exceed σcs.
Required profile coefficients Z1 and Z2 with respect to the upper margin and the lower margin of the profile of a girder having the above stress distribution should satisfy the following Equations 1 and 2.
                              Z          1                ≥                                                            (                                  1                  -                  R                                )                            ⁢                              M                d1                                      +                          M              d2                        +                          M              l                                                          σ              cs                        -                          R              ⁢                                                          ⁢                              σ                ti                                                                        [                  Equation          ⁢                                          ⁢          1                ]            
                              Z          2                ≥                                                            (                                  1                  -                  R                                )                            ⁢                              M                d1                                      +                          M              d2                        +                          M              l                                                          R              ⁢                                                          ⁢                              σ                ci                                      -                          σ              ts                                                          [                  Equation          ⁢                                          ⁢          2                ]            
In FIG. 2 and Equations 1 and 2, the required profile coefficient of the member according to the conventional design method in which prestress is introduced only once is calculated in consideration of the self-weight of the girder and additional dead and live loads. However, as the span increases, the bending moment due to the load increases in proportion to the square of the distance of the span. Accordingly, as the span of the girder increases, the profile of the girder increases. Then, the bending moment due to the self-weight further increases so that the member itself is made large. Therefore, though the profile of the member is deformed to improve the efficiency of withstanding stress, the aforesaid basic problem cannot be solved and this fact has been a great disadvantage in designing a long-span bridge using a PSC I-type girder.
All problems generated in a bridge can be solved by adjusting the tension of the girder used therefor. Thus, the present invention provides a solution which is simple and inexpensive.